1. Field of Invention
The present invention relates to a mobile communication system, more particularly, to a method for controlling a handoff in a mobile communication system so that its call quality and capacity are improved.
2. Background of the Related Art
In the mobile communication system using a code division multiple access (CDMA), some unnecessary handoffs emerge and then deteriorate the call quality. Therefore, a control method for the handoffs is required.
Generally, in order to keep the movability and the continuity of the communication in the CDMA mobile communication system, soft handoff or softer handoff has been used. The soft and softer handoffs are the mobile assisted handoff (MAHO) method based on the information that a mobile station provides. These soft and softer handoff methods consist of the process that the mobile station reports the information about the pilot signal strength of the base station and the process that the handoff control system (that is, a base station and a base station controller) performs the wireless channel resource assignment and release based on the information of the pilot signal strength of the base station.
During communicating a mobile station measures periodically the pilot signal strength of the active base set with which active communication path is established and the pilot signal strength of the candidate base station or the neighbor (or remaining) base station. If the measured pilot signal strength satisfied the handoff condition provided from the handoff control system, the mobile station attaches the corresponding measured result to a pilot strength measurement message (PSMM) and then sends the PSMM to the handoff control system. The handoff control system received the PSMM from the mobile station adds a new traffic path or drops the existing traffic path according to the strength information of the pilot signal included in the received PSMM.
Generally, the elements that is used by the mobile station to trigger the handoff are four parameters (T_ADD, T_DROP, T_COMP, T_TDROP). The parameter (T_ADD) is the threshold value of the pilot strength for the base station which is to be added to the traffic path. The parameter (T_COMP) is used for the additional generation of the PSMM. In other words, if the pilot signal strength of a base station which is measured by a mobile station is over the parameter (T_ADD), the mobile station reports the fact to the handoff control system through the PSMM. However, if the communication to the base station has not yet been established, the base station is managed as the candidate base station. The parameter (T_COMP) is used for the additional generation of the PSMM. On the other hand, the parameter (T_DROP) denotes the release condition for the currently active communication path and is the threshold value of the pilot signal strength to the base station which belongs to the communication path to be released. The parameter (T_TDROP) denotes the time threshold value which is the maximum strength of the pilot signal of the base station until the time just before the communication path presently established is released.
The conventional soft and softer handoff processes is explained by referring to FIGS. 1 and 2.
FIG. 1 is a diagram showing the pilot signal strength change of an arbitrary base station which is monitored, during communicating, by a active mobile station and the event according to the change. Referring to FIG. 1, the establishment and release of a new communication path between an arbitrary base station and a mobile station is explained.
First, referring to the time a1 in FIG. 1, since the presently measured strength of the pilot signal of a base station exceeds the parameter (T_ADD), the corresponding mobile station sends the PSMM to the handoff control system and moves the corresponding pilot from the neighbor set to the candidate set.
At the time a2 of FIG. 1, the handoff control system sends the mobile station the handoff direction message to establish a communication path with the corresponding base station. At the time a3 of FIG. 1, the mobile station receives the handoff direction message and moves the pilot of the corresponding base station from the candidate set to the active set and establishes a communication path with the corresponding base station. After then, the mobile station sends the handoff completion message to the handoff control system (Soft/Softer Add process).
However, at the time a4 in FIG. 1, if the measured strength of the pilot signal of the corresponding base station is smaller than the parameter (T_DROP), the mobile station transfers the value of the parameter (T_TDROP) into the handoff drop timer and then drives the timer. At the time a5 in FIG. 1, if the timer stops, the mobile station transmits the PSMM to the handoff control system.
Then, at the time a6 in FIG. 1, the handoff control system sends the mobile station the handoff direction message that is to release the communication path with the corresponding base station.
The mobile station, as shown at the time a7 in FIG. 1, moves the pilot of the corresponding base station from the active set to the neighbor set. In other words, the existing communication path with the corresponding base station is released and the mobile station sends the handoff control system the handoff completion message (Soft/Softer Drop process).
As described beforehand, a handoff starts by a mobile station. FIG. 2 is a diagram showing the PSMM generation condition according to the pilot of a candidate set. Therefore, FIG. 2 shows the condition for generating another PSMM when any response from the handoff control system or the direction to establish a communication path to the corresponding base station is not received after the mobile station sends the handoff control system the PSMM to add a new communication path. In other words, FIG. 2 shows the PSMM generation condition for a pilot when the pilot moves from the neighbor set to a candidate set but does not further move to an active set.
In FIG. 2, P0 is the strength of the pilot of the candidate base station. P1 and P2 are the strength of the pilot of the currently active base stations which have the communication path with a mobile station, respectively.
At the time t0 in FIG. 2, since P0 exceeds the parameter (T_ADD), the mobile station sends the handoff control system the PSMM for the pilot ol the candidate base station. At the time t1 in FIG. 2, since P0 is larger than P1 by the value (T_COMPx 0.5 dB), the mobile station sends the handoff control system the PSMM of the pilot of the candidate base station. At the time t2 in FIG. 2, since P0 is larger than P2 by the value (T_COMPxc3x970.5 dB), the mobile station sends the handoff control system the PSMM of the pilot of the candidate base station.
However, the conventional handoff method described above has the following problems.
First, in the process determining the necessity of the handoff, the conventional handoff method uses the strength of the pilot of the base station as a criterion. However, if only the pilot strength of a base station is used for the criterion, in the midtown circumstances in which the propagation situation is changed rapidly and widely according to the movement of the mobile station or to the changes of the surrounding interferences, the unnecessary handoffs that deteriorates the call quality and the system performance may be caused.
For example, if the pilot strength of a base station exceeds temporarily the parameter (T_ADD), the handoff can be caused. And, the communication path between the mobile station and the corresponding base station will be established. But, the established communication path will not serve as a real communication path and be released soon. The signaling between the handoff control system and the mobile station for the handoff does not have any meaning. But, the signaling deteriorates the call quality and increases the load of the communication system. Furthermore, the established communication path becomes only an interference to other subscribers.
Second, another communication path except the existing path is added and therefrom the diversity gain for the call can be increased. However, the diversity gain may be increased only in the case that the added path maintains the quality higher than a certain level. Particularly, if the existing established communication path maintains the enough quality, the diversity gain is not improved by adding a new communication path. In the CDMA mobile communication system, since adding a new communication path is equivalent to adding a new call in view of the forward link, adding a new communication path cases the system capacity drop in the case of the existing communication path with a good quality.
Third, if the average number of the communication paths which are established for a single call is higher than need, it mean that the hardware resources of the base station is wasted as much.
Therefore, those handoffs become the interference even to other subscriber and decrease the total system capacity.
Accordingly, the present invention is directed to provide a method for controlling a handoff in a mobile communication system by which unnecessary handoff is suppressed.
To achieve this and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the handoff controlling method uses, as the criterion elements for determining the necessity of the handoff, the pilot strength of the base stations as well as the information about the frame quality of the forward link (FER, BER, SER, etc.). As the other criterion elements, one of the pilot strengths of the currently active base stations is used as a relative threshold value to the pilot strengths of the other base stations each of which is to be a new active base station.
According to the present invention, the pilot strength of a base station and the frame quality of the forward link are measured by a motile station. The mobile station checks whether the measured pilot strength is higher than the parameter (T_ADD) that is a threshold value to the pilot strength of a base station to add a new communication path. It is checked whether the frame quality of the forward link is good based on the preset parameter (T_QUALITY). It is also checked whether the measured pilot strength is higher than the strength of the pilot in the active set that has T-PRANKth strength among active pilots. The mobile station sends the handoff control system the PSMM in order to establish a new communication path by using the base station according to the checked results.
On the other hand, as the checked results, if the frame quality of the forward link is good based on said parameter (T_QUALITY) but the measured pilot strength is higher than the parameters (T_ADD, T_PRANK), the mobile station sends the PSMM to the handoff control system.
However, even if the frame quality of the forward link is not good based on said parameter (T_QUALITY) but the measured pilot strength is higher than the parameters (T_ADD, T_PRANK), the mobile station can send the PSMM to the handoff control system.
Thus, according to the present invention, the unnecessary handoffs are reduced. Furthermore, the capability of the total mobile communication system is increased. The frequent repeat of the communication path add and drop is reduced and therefore the interference effects to other subscribers are prohibited. Since the frequent signaling between the mobile station and the handoff control system is prohibited, the deterioration of the call quality is prohibited and therefore the load of the mobile communication system is reduced.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.